The present invention relates generally to fundus and ocular cameras and indirect ophthalmoscopes used by ophthalmologists or ophthalmic photographers to capture and record images of the ocular and retinal anatomy used in diagnosis of ocular and retinal abnormalities. More particularly, the present invention relates to a combined apparatus for the capture, processing and archival recording of digital or analog images of the ocular and retinal anatomy.
Generally there are three methods used to photographically document the retinal fundus with an eye fundus camera. The first method is to take a picture of the eye fundus using visible light, the second method is to take a picture of the eye fundus vasculature using a fluorescent dye, known as fluorescein angiography and the third method is to take a picture of the eye choroidal vasculature using and infrared light stimulated dye, known as indocyanine green angiography.
Fluorescein angiography is a widely used method in which a fluorescent dye, typically sodium fluorescein (C.sub.20 H.sub.12 O.sub.5 Na), and more recently indocyanine green, is administered intravascularly to the patient and the eye fundus is exposed to light energy to excite the fluorescein in the eye fundus vasculature. Excitation of the fluorescein causes a fluorescence which is visible to the practitioner when using certain optical filters and may be recorded by photography.
Fluorescence is luminescence which is maintained only by exposure to a continuous excitatory energy. Fluorescence is emission of light immediately after excitation and cessation of emission immediately after cessation of excitatory radiation. Luminescence refers generally to the emission of light due to any cause other than high temperature. The second law of thermodynamics dictates that emitted energy must be less than the energy absorbed. Thus, since energy and wavelength are reciprocally related, luminescence, and, hence, fluorescence, always entails a shift from a shorter wavelength, i.e., higher energy, in the excitation radiation to a longer wavelength, i.e., lower energy, in the emitted light.
Sodium fluorescein, for example, in solution at proper concentration and pH, is excited by light energy between 465 to 490 nm in the blue portion of the light spectrum, and fluoresces at a peak wavelength of 520 to 530 nm in the green-yellow portion of the light spectrum. As with all known fluorescent materials, sodium fluorescein has an energy absorption curve which decreases in the shorter wavelength and rapidly in the longer wavelengths, and a fluorescence which rises rapidly over the shorter wavelengths and diminishes slowly over the longer wavelengths. Sodium fluorescein is known to fluoresce over a spectral curve range of 485 to 600 nm.
Fluorescein angiography is typically conducted by intravenously injecting sodium fluorescein into an arm vein of the subject to be tested. The normal fluorescein angiogram can be divided generally into the following phases:
i. early choroidal filling and choroidal flush; PA0 ii. retinal artery filling and increased choroidal filling; PA0 iii. arterio-venous filling and laminar flow; PA0 iv. full arterio-venous filling; PA0 v. retinal venous phase; and PA0 vi. late arterio-venous recirculation phase with decreased retinal and choroidal fluorescence.
It usually takes about 5 to 10 seconds before the fluorescein enters the eye fundus vasculature. The early signs of fluorescence is referred to as the "choroidal flush" due to entry of the fluorescein into the choroid. This choroidal fluorescence is seen because unbound fluorescein molecules pass through the fenestra of the choriocapillaris and fill the extracellular choroidal space. One to two seconds after the choroidal flush is noted, fluorescence appears in the central retinal artery and the larger precapillary arteriole branches. The fluorescein then passes into the retinal capillaries, the post capillary venules and the major retinal veins and the central retinal vein. The early phase of the retinal venous fluorescein pattern is often referred to as laminar flow. This laminar flow provides a characteristic picture because the vascular flow is faster in the center of the larger retinal veins than on the sides. The fluorescence along the walls of the veins becomes thicker, and eventually there is a complete fluorescence within the lumen of the vein. Fluorescence of the disc originates from the posterior ciliary vascular system and from the capillaries of the central retinal artery on the surface of the disc. The macular region of the normal fluorescein angiogram characteristically has a darker appearance than the surrounding region. Xanthophyll in the sensory retina partially blocks blue light transmission needed to excite the fluorescein in the choroid. Additionally, the increased density of the melanin pigment granules in the retinal pigment epithelium underlying the macula also block some of the choroidal fluorescence during fluorescein fundus angiography. In the case of indocyanine green fundus angiography, both the infrared excitation wavelength and infrared fluorescence wavelength easily pass through the xanthophyll and melanine pigment layers to reveal details of the choriodal vasculature layer.
During fluorescein angiography, a flash of white light from a retinal fundus camera passes through a blue excitation optical filter which passes blue light having a peak wavelength of 465 to 490 nm and strikes the fluorescein molecules in the ocular vasculature. The blue light excites the fluorescein molecules which fluoresce and emit a yellow green light with a peak wavelength of 520 to 530 nm. Both yellow-green light as well as reflected blue light emerges from the patient's eye. A yellow-green optical barrier filter is used to block the blue light and transmit only the yellow-green wavelengths onto camera film and to the viewing oculars.
As noted above, angiography of the eye fundus typically employs sodium fluorescein dye as the imaging medium. Information concerning the dynamics of retinal and choroidal circulation have been derived principally from fluorescein angiography. Except for the earliest choroidal arterial filling, i.e., the choroidal flush, visualization of the choroidal circulation is limited by both the spectral characteristics of the eye pigments and tissue and the rapid extravasation of fluorescein from the choriocapillaris.
As noted by Hochheimer, et al., U.S. Pat. No. 3,893,447, choroidal circulation may be visualized separately from the retinal circulation by using indocyanine green dye. The methods and principles concerning indocyanine green fundus angiography are essentially identical to fluorescein fundus angiography. The principal difference with fluorescein fundus angiography is that indocyanine green fluoresces in the infrared spectrum and allows visualization of the choroidal circulation dynamics on infrared film or by and infrared detector. As taught by Hochheimer, sodium fluorescein may be mixed with indocyanine green and the mixture injected intravenously. Angiograms of the separate circulation are simultaneously produced by two cameras mounted on a fundus camera equipped with an optical separation device. The light energy returning from the ocular fundus during each flash firing is split by the optical separator into two or more discrete beams. One split beam corresponds to the spectral range of the sodium fluorescein fluorescence, i.e., 490-520 nm, in the retinal circulation, while the second split beam corresponds to the absorption spectrum of indocyanine green, which is near 800 nm, in the choroidal circulation. In the infrared spectrum at about 800 nm, macular xanthophyll and the pigment epithelium are relatively transparent and energy absorption by indocyanine green is detectable. The two cameras are equipped with appropriate optical filters to pass only the yellow-green light of the sodium fluorescein or the infrared light of the indocyanine green dye.
Fluorescein angiography typically employs two optical filters; an exciter filter and a barrier filter. The exciter filter transmits blue light at 465 nm to 490 nm, the absorption peak of fluorescein excitation. The barrier filter transmits light at 525 to 530 nm, the fluorescent peak of fluorescein. Optimally, there should be little or no overlap between the filter curves to eliminate pseudofluorescence. Pseudofluorescence is non-fluorescent light which passes through both the exciter and barrier filters. Pseudofluorescent light records onto black and white film and results in reduced contrast and artefactual fluorescence. Conventional optical filters are available as matched sets from Baird-Atomic, Spectrotech and De Lori.
Imaging of the peripheral retina using standard fundus cameras is a difficult task which requires a high degree of skill and practice. Problems with patient position, alignment and focusing are compounded by marginal corneal astigmatism, unsteady patients, light reflexes and awkward camera displacement. While various cameras employ different compensating mechanisms, peripheral imaging remains a significant shortcoming of conventional fundus cameras.
Finally, current fundus cameras employ a variety of films to record the fluorescent light emanating from the retina. The film most frequently used in Kodak "TRI-X" film which is a fairly fast film of ASA 400. Angiographers also employ a variety of different film development techniques which enhance detail but compromise contrast. The developed negatives or prints made from the resulting negatives are often enclosed in patient record files. Videocameras may be employed in place of the film camera as illustrated by European Patent Application No. 153,570 published Jan. 16, 1985.
Indirect ophthalmoscopy is a method which permits visualization of the peripheral retinal area. Examples of indirect ophthalmoscopes and methods of ophthalmoscopy are provided by U.S. Pat. No. 4,146,310 to Kohayakawa, Y., et al., U.S. Pat. No. 4,018,514 to Plummer, and U.S. Pat. No. 3,881,812 to Ben-Tovim. Each of these systems employ a lamp, a mirror worn on a harness placed on the observer's head, and a lens which the observer must hold in front of the eye to be examined. Use of indirect ophthalmoscopes requires positioning of the light relative to the both the observer's and subject's eye, positioning the mirror at a proper angle and placing the lens at a defined distance from the subject's eye and tilted to exclude reflexes. The retina is observed through the lens held in front of the patient's eye. A film or video camera is sometimes used in conjuction with the indirect ophthalmoscope to photographically record the retinal image.
Current fundus cameras cost tens of thousands of dollars and add-on video cameras or image processing equipment only adds to the total system cost. Moreover, processing and manipulating the current film or video images requires cumbersome and expensive digitization as a separate process which further increases cost and introduces a time delay into the diagnostic process. With the advent of floppy disc based video and still cameras, direct analog and digital processing of the retinal images obtained by either a fundus camera or an indirect ophthalmoscope is possible. Moreover, the use of optical barrier filters requires careful matching to the exciter filter to minimize pseudofluorescence. There is, however, usually some remaining spectral overlap which tends to degrade image quality. Electronic imaging processing permits the elimination of the barrier filter by electronically filtering all wavelengths except that desired for imaging and recording purposes.